Download Distribution and formation conditions of gibbsite in the upland soils

Distribution and formation conditions of gibbsite in the upland soils of humid
Asia: Japan, Thailand and Indonesia
Tetsuhiro WatanabeA, Shinya FunakawaB and Takashi KosakiC
A
Graduate School of Agriculture, Kyoto University, Kyoto, Japan, Email [email protected]
Graduate School of Agriculture, Kyoto University, Kyoto, Japan, Email [email protected]
C
Graduate School of Urban Environmental Sciences, Tokyo Metropolitan University, Tokyo, Japan, Email [email protected]
B
Abstract
Gibbsite is considered to be an ultimate product of weathering. However, soils in temperate regions such as
Japan usually contain gibbsite, whereas some strongly weathered soils in tropical regions do not. We
investigated gibbsite distribution in humid Asia (Japan, Thailand and Indonesia) and the relationship
between gibbsite and soil solution to explain distribution according to soil forming factors. Clay mineralogy
was determined by X-ray diffraction and differential thermal analyses. The chemical composition of soil
water extract (indicative of field soil solution) was determined to evaluate minerals’ thermodynamic
stability. Relatively large amounts of gibbsite were found in the Japanese soils, and no or small amounts in
the soils from Thailand and the Java-Sumatra and East Kalimantan region of Indonesia. In Japanese soils the
distribution of gibbsite, in which H4SiO40 activity was high enough to be in the kaolinite stability field, was
explained by 1) rapid precipitation or formation of gibbsite, and its conservation due to low temperature,
which retards resilication to more stable kaolinite under a given H4SiO40 activity, or 2) intense leaching
resulting in low H4SiO40 activity. In Thailand and the Java-Sumatra region, high temperatures may enhance
formation of the more stable minerals kaolinite and smectite. In contrast to Thai and Indonesian soils,
Japanese soils are characterized by an abundance of Al hydroxides or gibbsite, as well as amorphous Al
hydroxides and interlayered Al hydroxides between 2:1 layers.
Key Words
H4SiO40 activity; stability diagram; soil solution; weathering.
Introduction
It is widely accepted that gibbsite is “the ultimate end product of weathering”, and is usually present in high
concentrations in old and weathered soils such as Oxisols and Ultisols. Gibbsite is also found commonly in
young soils, i.e. Inceptisols and Andisols. The occurrence of gibbsite in these young soils is usually ascribed
to the rapid desilication that occurs in very permeable materials under high rainfall or to paleoclimate. In
contrast, Green and Eden (1971), Herrmann et al. (2007) and Vazquez (1981) have determined gibbsite
occurrence in soils in the initial stages of weathering in southwest England, northern Spain and northern
Thailand, respectively.
In humid Asian regions such as Japan, Thailand and Indonesia, information on gibbsite distribution is still
scarce. The geological conditions of these regions, characterized by steep topography caused by intense
orogenesis and small areas of Oxisols, differ significantly from those of the South American and African
shield areas, and differences in gibbsite distribution patterns is to be expected. Characterizing gibbsite
distribution and formation conditions will deepen our understanding of soils in these regions.
Thermodynamic analysis and weathering indices help to elucidate the formation conditions of clay minerals.
Thermodynamic analysis of soil solution is useful for the theoretical understanding of clay mineral
distributions and weathering trends (Watanabe et al. 2006). H4SiO40 activity controls mainly the relative
stability of gibbsite, kaolinite and smectite: gibbsite is most stable under strong leaching conditions with low
H4SiO40 activity, whereas kaolinite is stable under moderate H4SiO40 activity conditions and smectite is
stable under restricted drainage with high H4SiO40 activity conditions. Among soil weathering indices, total
reserve in bases (TRB) and the relative content of crystalline iron oxides are most common.
The objective of this study is to investigate soil gibbsite distribution in extensive regions of Japan, Thailand
and Indonesia, and to discuss gibbsite formation conditions through thermodynamic analysis and weathering
indices, which will increase our understanding of soils in these regions.
Materials and methods
Soils
We collected soil samples from Japan, Thailand and Indonesia, according to the soil distribution patterns in
the respective regions based on geology and climate. Climatic conditions differ between the regions and soil
© 2010 19th World Congress of Soil Science, Soil Solutions for a Changing World
1 – 6 August 2010, Brisbane, Australia. Published on DVD.
17
temperature and moisture regimes are mesic/thermic-udic in Japan, isohyperthermic-ustic in Thailand, and
isohyperthermic-udic in Indonesia. Soil parent materials reflect their geological conditions. Parent materials
of soils from Japan, where volcanic belts are distributed, include felsic igneous and sedimentary rocks, and
tephra. The soils in Thailand are formed on sedimentary and felsic igneous rocks. The soils from Java and
Sumatra, where volcanic belts are also distributed, are formed on andesite, tephra, sedimentary rocks, and
limited areas of felsic rocks. Most of East Kalimantan, Indonesia is underlain by sedimentary rocks, there are
no active volcanoes, and soils are more weathered on stable rolling landscapes.
Types of soils investigated were common in each region. Japanese soils consist mostly of Udepts, Thai soils
Ustults and Humults, Java-Sumatra’s Udepts and Udults, and East Kalimantan’s Udults. Oxisols are
restricted to very limited areas of humid Asia and were not included in the samples.
Analytical methods
Clay minerals were identified by X-ray diffraction (XRD). Gibbsite and kaolin minerals in the clay fraction
were quantified by differential thermal analysis (DTA). Before DTA, iron oxides were removed by citratedithionite-bicarbonate treatments. Fe (Feo) was extracted with ammonium oxalate in the dark. Fe (Fed) was
also extracted with citrate-dithionite-bicarbonate. Total elemental analysis was made with HF and HClO.
The weathering indices, TRB and (Fed-Feo)/Fetotal ratio were calculated; TRB is the sum of basic cations
(Ca+Mg+K+Na) and gives a chemical estimation of weatherable minerals, and the (Fed-Feo)/Fetotal ratio
indicates the relative content of crystalline iron oxides.
For thermodynamic analysis on the stability of minerals, soil water extracts, of which composition gives an
indication of soil solution composition in the field, were collected by continuously shaking the soils for 1
week at 25°C and 1 atm with a soil to water ratio of 1:2. In these samples, pH and Si, Al, Ca, Mg, Na, K
concentrations were determined. Inorganic monomeric Al species were determined by the method of Driscoll
(1984). Ionic activities were calculated by the extended Debye-Hükel equation.
Results
Gibbsite detection by XRD and DTA
Results of XRD show that gibbsite was detected in most Japanese soils, whereas a gibbsite peak was usually
very small or not detected in the other study regions (Figure 1a). In XRD analysis, a large part of Al
hydroxide between the 2:1 layers remained even after heat treatment up to 350°C for Japanese, Thai and
Java-Sumatra samples, while a small part remained for the East Kalimantan ones. The gibbsite content
measured by DTA was high in Japanese samples, low in Thai and Java-Sumatra samples, and not detected in
most East Kalimantan soils (Figure 1b).
b)
350
Gibbsite
Japan
(37/42 sites)
Thailand
(44/92 sites)
Java-Sumatra
(8/38 sites)
E. Kalimantan
(14/52 sites)
-1
Expandable
Kaolin
2:1 type
Minerals
Mica
Gibbsite content (g kgclay )
a)
300
250
200
150
100
50
0
1.42
1.0
0.72
(nm)
0.485
Japan
Thailand
Java- E.
Sumatra Kalimantan
Figure 1. Gibbsite detection by X-ray diffraction (a) and differential thermal analysis (b). Number of sites where
gibbsite was detected by X-ray diffraction is in the parentheses.
Weathering indices
Weathering indices indicated that Japanese and Java-Sumatra soils were younger than those in Thailand and
East Kalimantan (Figure 2). The (Fed-Feo)/Fetotal ratio, indicating the relative content of crystalline iron
oxides, was low in Japanese and Java-Sumatra soils (mostly less than 0.6), and high in Thai and East
© 2010 19th World Congress of Soil Science, Soil Solutions for a Changing World
1 – 6 August 2010, Brisbane, Australia. Published on DVD.
18
(Fed-Feo)/Fetotal
Kalimantan soils (mostly more than 0.6). TRB values, giving a chemical estimation of weatherable minerals,
were high in Japanese, Java-Sumatra and Thai soils, with averages of 128, 136 and 106 cmolc kg-1,
respectively. The TRB values in East Kalimantan soils were smaller than in the other regions, with an
average of 69 cmolc kg-1. The high values of TRB (Figure 2) and presence of 2:1 type minerals (Figure 1a)
indicate that all the investigated soils still contain some amount of weatherable minerals, and are not as
strongly weathered and desilicated as Oxisols or Ferralsols.
1.0
Japan
0.8
Thailand
0.6
Java-Sumatra,
Indonesia
0.4
E. Kalimantan,
Indonesia
0.2
0.0
0
100
200
300
400
500
Total reserve in bases (cmolc kg-1)
Figure 2. Weathering indices of the soils: total reserve in bases and relative content of crystalline iron oxides.
Composition of soil water extract
Most of the water extracts for Japan, Thailand and Java-Sumatra were supersaturated with gibbsite,
indicating the possibility of gibbsite precipitation (Figure 3a). In contrast, East Kalimantan water extracts
were undersaturated, indicating dissolution or non formation of gibbsite. Judging from H4SiO40 activity in
the soil water extracts, kaolinite was considered to be the most stable mineral in most of the soils (Figure 3b).
For the Java-Sumatra region, H4SiO40 activity in the soil water extracts was high and the extract
compositions are close to or within the stablity field of smectite (Figure 3b). Composition of soil water
extract indicated that gibbsite can form in Japanese soils where the composition is supersaturated with
gibbsite (Figure 3a), but is unstable compared with kaolinite (Figure 3b).
a)
b)
-2
9
Thailand
7
Amorph. Al(OH)3
-6
pH
log (Al-OHtotal)
8
-4
Japan
Smectite
stability field
6
5
Gibbsite
-8
Java-Sumatra,
Indonesia
E. Kalimantan,
Indonesia
4
Gibbsite
stability
field
Kaolinite
stability
field
3
3
4
5
6
pH
7
8
-5.0
-4.5
-4.0
-3.5
log (H4SiO40)
-3.0
-2.5
Figure 3. Composition of soil water extract plotted on a solubility diagram with gibbsite and amorphous Al(OH)3
solubility lines (a) and a stability diagram representing relative stability of gibbsite, kaolinite and smectite.
Discussion
Gibbsite formation conditions in humid Asia
The presence of gibbsite in weakly weathered soils where soil water extract indicated that kaolinite or
smectite is most stable can be explained by 1) a higher precipitation rate of gibbsite as a transient mineral
than that of kaolinite in soil solutions supersaturated with gibbsite, or 2) low H4SiO40 activity due to rapid
removal of soil solution or a high leaching rate. Both explain the formation of gibbsite in young soils that are
not strongly desilicated. The former is demonstrated by May et al. (1986) as gibbsite formation under high
H4SiO40 activity of the smectite stable range, which is explained by Gay-Lussac-Ostwald Step Rule (Sposito
1994). Synthesis of gibbsite in laboratory experiments is easy and rapid compared to that of kaolinite (Nagy
1995). The second factor is assumed in weathering saprolites and soils derived from volcanic ash (Huang et
al. 2002).
© 2010 19th World Congress of Soil Science, Soil Solutions for a Changing World
1 – 6 August 2010, Brisbane, Australia. Published on DVD.
19
Depending on gibbsite’s precipitation rate, the regional differences in the amount of gibbsite between Japan
and the tropical regions of Thailand and Java-Sumatra were presumably due to the difference in
temperatures: high temperature in tropical regions stimulates chemical reactions and results in dominance of
the most stable mineral or kaolinite, which may consume gibbsite, if present, precipitated as less stable
mineral. Thus, the lifetime of gibbsite in tropical regions with high H4SiO40 activity, where kaolinite or
smectite is stable, is short. On the other hand, the low temperatures in Japan are considered to inhibit the
formation of kaolinite, and gibbsite remains as a transient mineral.
Rapid removal of soil solution resulting in low H4SiO40 activity did not explain well the regional distribution
pattern of gibbsite. The content of gibbsite in Japanese soils is higher than that in Indonesian soils, although
the leaching in Indonesian soils seems to be as strong as or stronger than Japanese soils.
The above explanations for the presence of gibbsite in young soils under cool temperature conditions seen in
this study cannot be applied to the presence of gibbsite in Oxisols. The presence of gibbsite in highly
weathered soils such as Oxisols on the shields in South America and Africa is supposed to be caused by
strong desilication, where gibbsite is present as the most stable mineral.
Importance of Al hydroxides in Japanese soils
The occurrence of gibbsite as well as the other Al hydroxides, i.e. amorphous Al hydroxides and interlayered
Al hydroxides (Watanabe et al. 2006) characterize Japanese soils in comparison to Indonesian and Thai soils.
These Al hydroxides affect soil properties; amorphous Al and gibbsite contribute to acid neutralization in
soils (Watanabe et al. 2008), cancel the negative charge in 2:1 layers (Funakawa et al. 2008), adsorb
phosphorus (Huang et al. 2002) and accumulate soil organic matter (Imaya et al. 2007).
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© 2010 19th World Congress of Soil Science, Soil Solutions for a Changing World
1 – 6 August 2010, Brisbane, Australia. Published on DVD.
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